Transesterification reaction and morphology of PET/PEN melt-mixed blends and their flexural strengths

Poly(ethylene terephthalate) (PET)/poly(ethylene naphthalate) (PEN) blends with 50150 composition have been prepared by melt-mixing at the temperatures above 280degreesC and subsequent heating at 280degreesC before hot-press molding. Progress of transesterification reaction and formation of PET-PEN random copolymers during these melt-blending processes have been confirmed by H-1-NMR measurement. Dynamic viscoelasticity, DSC thermograms, and SEM images for the PET/PEN blends with shorter heating time at 280degreesC suggested that the blend sample was in an immiscible state when the transesterification level (T) was less than 5%. With the increase of the heating time, the sample changed to miscible state with T > 5% and exhibited transparency. The phase-separated morphology was fine and the size of the phase-separated domain was several hundred nanometer when T approximate to 5%. Flexural test for the blend samples revealed that the PET/PEN blends exhibited the maximum flexural strength when T approximate to 5%. Increase in the melt-mixing temperature was found to accelerate the transesterification reaction, and to shorten the processing time to obtain the miscible PET/PEN blend.